1. Field of the Invention
The present invention relates to compositions and methods for forming flexible ceramic articles termed greensheets and, in particular, thin greensheets having a thickness of about 10-75 microns and for forming the greensheets into multilayer ceramic (MLC) substrates including MLC substrates having an interlayer capacitor and, in particular, to a composition comprising a ceramic having a defined particle size and particle size distribution and to a specially defined heating (sintering) profile to form the MLC which heating profile effectively removes the organics used to make the greensheets.
2. Description of Related Art
Ceramics have found widespread use in electronics as a substrate for integrated circuit packages. Metallized circuit patterns are applied to the ceramic substrate in the form of a greensheet, and ceramic and metallization are co-sintered to create a monolith of substrate and circuitry. Multi-layer circuit packages are constructed by combining ceramic particles and organic binders into unfired, or "greensheet", tape. Inter-layer conductive paths, known as "vias", are then inserted (punched) through the layers, forming electrical interconnections between the circuits on each layer after they are stacked and processed. Metallized circuit patterns are applied to the punched greensheets as is well-known in the art. The greensheet tape layers typically have a thickness ranging from 125 to 625 microns. Multiple layers of punched and metallized tape are stacked and laminated under pressure. The ceramic and metallization laminate is then co-sintered to form a monolithic structure with three-dimensional circuitry.
The casting of suspensions of ceramic material to form layers or sheets which are then sintered to produce a ceramic substrate material is known in the art. The doctor blade method is one method for producing a ceramic greensheet. Typically, ceramic powder is mixed with an organic solvent, a plasticizer and a binder forming a slurry, the slurry is cast in a regulated thickness on a carrier film with the aid of a doctor blade, and the applied layer of the slurry is then dried. The ceramic formula is typically alumina and a butyral type resin like polyvinyl butyral. A cellulose type resin like ethyl cellulose or polyvinyl alcohol is also typically used as the binder.
A capacitor can be formed within the multilayer substrate by sandwiching an inner dielectric layer between a pair of electrodes. In such an integrated capacitor substrate, conductor pads are formed on the top sheet and wirings are formed within the substrate to connect the capacitor and the pads. U.S. Pat. Nos. 4,567,542; 5,065,275 and 5,099,388 show such MLC's having internal capacitors and the patents are hereby incorporated by reference.
The capacitor containing multilayer substrate is likewise manufactured by a greensheet laminating technique as described above. Alumina and glass are typically used as the ceramic component of the greensheet for both the insulating layers and dielectric layers. The laminated greensheets are typically sintered or fired at a temperature of about 1,500.degree. C. or higher. In view of the high sintering temperature, the vias and wiring must be made of molybdenum, tungsten, or a like conductor material having a high melting point.
In order to build a capacitor within the MLC to provide the electrical capacitance, a dielectric layer must be placed between two electrode layers. Since the capacitance is inversely proportional to the thickness of the dielectric layer, a thin dielectric layer is required for higher capacitance. The thinner a MLC dielectric greensheet is, however, the more difficult it is to process. Some of the processes that a greensheet needs to undergo to fabricate an MLC are spooling after casting, blanking greensheets from the cast tape, via punching, greensheet stacking, and the handling of the tape/sheets during each process. A thin tape made of conventional materials yields a tape that is very difficult to blank into greensheets. The greensheets are difficult to process at punching and stacking since they rip or tear due to their poor mechanical properties. A greensheet having high strength and flexibility is therefore very desirable.
Another problem arises when the layers have different ceramic compositions such as the dielectric ceramic layer used to form the capacitor and the ceramic and glass layers of the multilayer substrate. When different ceramic material layers are laminated and sintered to form MLC, it is typically termed co-sintering. Problems such as blistering can occur during co-sintering due to the release of gaseous organic materials formed during removal of the binder and other organics from the greensheet layer. Basically, the gases collect and the volume of the gas increases as the temperatures rise during the sintering process. The result is the formation of blisters in the thin layer material causing surface perturbations making the surface of the MLC non-planar and, therefore, unacceptable.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a ceramic greensheet casting composition which provides greensheets having enhanced strength and flexibility properties.
It is another object of the present invention to provide a ceramic greensheet having enhanced strength and flexibility properties.
A further object of the invention is to provide a process for producing a ceramic substrate greensheet wherein the greensheet has enhanced strength and flexibility properties.
It is yet another object of the present invention to provide a process for fabricating a multilayer ceramic substrate from greensheets including multilayer ceramic substrates having an inner capacitor.
It is still yet another object of the present invention to provide a process for fabricating a multilayer ceramic substrate having the same or different ceramic composition greensheet layers using a specially defined heating procedure which minimizes the formation of blisters in the multilayer ceramic product.
Another object of the present invention is to provide greensheets and multilayer ceramic substrates made from the greensheets using the compositions and/or processes of the invention.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.